This invention relates to a novel colorless dye (precursor) suitable for use in image recording materials which utilize a color developing reaction between an electron-donating colorless dye and an electron-accepting color developer, such as, for example, carbonless pressure-sensitive recording materials, thermosensitive recording materials, and other image recording materials.
As the colorless dyes used heretobefore for the above-mentioned purpose, there may be mentioned so-called lactone dyes including colorless dye of the triphenylmethane lactone type such as typically 3,3-bis(4-dimethylaminophenyl-6-dimethylaminophthalide (so-called Crystal Violet Lactone) and those of the fluorane lactone type such as typically 3-diethylamino-6-methyl-7-anilinofluorane. These lactone dyes are well-known as primary dyes (colorless dyes capable of developing colors immediately upon contact with acid substances which act as color developers).
On the other hand, dyes in reduced form and acylated derivatives thereof such as benzoyl-leucoMethylene Blue and carbazolylmethane compounds described in Japanese Patent Publication No. 16,358/85 can also be used as colorless dye. These dyes, however, do not form color unless brought into contact with developers of high oxidizing power (inorganic solid acids such as acid clays or activated clays are usually used). Moreover, even when the contact is made, an extended period of time is required before color is slowly developed (such colorless dyes are called secondary dyes). The secondary dyes do not form colors with the well-known developers based on organic acid substances such as phenolic compounds or polyvalent metal salts of salicylic acid.
Crystal Violet Lactone has been generally used as a primary dye, especially as that which gives blue-colored images. The color image formed from Crystal Violet lactone is susceptible to fading, especially fading caused by light or a plasticizer. As a consequence, there has been a strong request for the improvement in color fastness of primary dyes.
The colorless dye provided by the present invention are a group of novel compounds not described in the literature.
As described above, there has been known no colorless dye to be used as a primary dye capable of forming an image in fast blue color by momentarily reacting with various common developers (acid substances).